Do you want to disallow multiple uses of the same authentication token? This restricts you to one login about every 30s, but it increases your chances to notice or even prevent man-in-the-middle attacks (y/n) y

By default, tokens are good for 30 seconds and in order to compensate for possible time-skew between the client and the server, we allow an extra token before and after the current time. If you experience problems with poor time synchronization, you can increase the window from its default size of 1:30min to about 4min. Do you want to do so (y/n) n

If the computer that you are logging into isn't hardened against brute-force login attempts, you can enable rate-limiting for the authentication module. By default, this limits attackers to no more than 3 login attempts every 30s. Do you want to enable rate-limiting (y/n) y

Following the link in the text above displays a QR code image in the web browser.

This image can be scanned with a smartphone or tablet camera with the Google Authenticator app.

The next time the system is logged into, a Google Authenticator verification code will be required:

nullock tells PAM to accept null if the user does not have the Google Authenticator configured. In other words, users without dual-factor configured can still log in.

secret= ... gives PAM access to the needed key file, even with SELinux installed. After that, the file .google_authenticator must be moved to the .ssh folder of the user. The folder may need to be created. It is best to do this as the user.